(1) Field of the Invention
This invention concerns size reduction machines, and in particular to a screen holder for use with a size reduction machine that positions and compressively locks a screen associated with the size reduction machine in place. This invention also relates to a mechanism for setting the gap between the impeller and the screen of a size reduction machine. This invention further relates to size reduction machines that can be easily disassembled for cleaning.
(2) Description of the Art
Maintaining the gap between the impeller and the screen of a size reduction machine is important in controlling product particle size. Therefore, it is imperative that the gap dividing the size reduction machine impeller from the size reduction machine screen is held constant during size reduction machine use. Furthermore, since a variety of screen sizes and impeller designs can be used within a single size reduction machine to produce products having a wide range of particle sizes, it additionally becomes important to be able to consistently adjust the gap between the size reduction machine screen and the size reduction machine impeller to control product particle size. Being able to adjust the impeller/screen gap is also important to maintain geometric screen uniformity because any non-uniformity such as warpage can detrimentally effect product particle size and/or particle size distribution.
Some size reduction machines of the prior art use frusto-conical shaped screens located in a channel between an input and an output. Such a size reduction machine is disclosed, for example, in U.S. Pat. No. 4,759,507, which describes using various screen openings of varying size and shape and using various impeller types to control particle size. According to the ""507 patent, once a screen and impeller have been selected, the operation and efficiency of the machine depends upon the gap between the impeller and the interior wall surface of the screen. With the ""507 patent device, different wall thickness screens are compensated for by inserting or removing spacers on the impeller shaft in order to move the impeller relative to the interior wall surface of the screen. Since the wall of the screen is tapered relative to the impeller, the actual adjustment of the gap is less than the thickness of the spacer and depends upon the angle of the screen relative to the horizontal. Where the tapered wall of the screen has an angle of sixty degrees relative to the horizontal, the gap is adjusted by one half the thickness of the spacer.
The use of spacers to control the screen/impeller gap creates difficulties. The process of installing a spacer and repeatedly removing and replacing incremental spacers is time consuming. Further, since the spacers must be incrementally sized and machined, the cost of producing such spacers is relatively high. Spacers are also easily lost during cleaning which can lead to re-assembly of the size reduction machine with an improper gap setting and decreased performance.
Adjustable size reduction machines without spacers are known in the art. For example, U.S. Pat. Nos. 4,773,599, 4,759,507, and 4,768,722 disclose machines in which the gap between the impeller and the screen is determined by the thickness of the screen flange or in which the gap is set by indexing the axial position of the impeller shaft when the machine is not in operation.
U.S. Pat. No. 5,282,579 discloses a size reduction machine with an adjustable impeller shaft. The impeller shaft is constructed in two parts that are united by a spacer device that operates much like a caliper to adjust the impeller shaft length, and thereby the gap between the impeller and the screen. One problem with the ""579 patent device is that the gap cannot be adjusted after the size reduction machine is assembled.
U.S. Pat. No. 4,605,173 discloses a size reduction machine with an adjustable stop for limiting the maximum travel of the impeller into the frusto-conical screen. U.S. Pat. No. 5,505,392 discloses a size reduction machine having an adjustable length rotary drive coupling. The coupling includes two rotary shafts, one of which has at least one tooth and the second of which includes varying depth abutment surfaces. The union of a tooth with an abutment surface sets the gap between the impeller and the frusto-conical screen of the size reduction machine.
Despite the advances made in size reduction machine design, there remains a need for size reduction machines with improved mechanisms for setting the impeller to screen gap. Specifically, there remains a need to be able to provide simple, positive and accurate incremental adjustment of the screen to impeller gap without disassembling the size reduction machine. In addition there is a need for devices that maintain geometry of a frusto-conical screen and its concentric alignment with respect to the impeller shaft. There is further a need for a size reduction machine that is easily disassembled for cleaning.
It is an object of this invention to provide a screen holder for a size reduction machine that maintains the geometric integrity of a size reduction machine screen.
It is another object of this invention to provide a screen holder that is able to compressively secure a size reduction machine screen in an axial position.
A further object of this invention is to provide a screen holder that is able to concentrically position the screen with respect to the impeller shaft.
Yet another object of this invention is a screen holder that re-forms or re-shapes warped screens.
It is a further object of this invention to provide a mechanism for easily adjusting and setting the gap between the impeller and screen associated with a size reduction machine. Yet another object of this invention is to provide a positive, incremental, reproducible and known gap between impeller and screen associated with a size reduction machine.
In one embodiment, this invention is a screen holder for use with a size reduction machine. The screen holder comprises several elements including a first flange having a top surface, a bottom surface and an opening wherein the first flange bottom surface includes a screen pilot. The screen holder further includes a second flange having a second opening. Finally, the screen holder includes at least one support arm uniting the first flange with the second flange.
In another embodiment, this invention includes an adjustable impeller for use with a size reduction machine. The adjustable impeller comprises several elements including an impeller having at least one arm and a hub including central aperture wherein the central aperture includes a threaded portion. The adjustable impeller further includes an impeller drive shaft associated with the drive mechanism and having a first end and a second end associated with the drive housing wherein the impeller drive shaft includes a threaded portion that is complementary to the impeller central aperture threaded portion.
In yet another embodiment, this invention includes a method for setting a gap between an impeller and a frusto-conical screen of a size reduction machine where the size reduction machine includes an impeller drive shaft, and a drive mechanism. The gap is set by rotating the impeller which includes at least one arm attached to a hub having a central aperture that further includes a threaded portion in relation to an impeller drive shaft having a first end and a second end associated with the drive mechanism wherein the impeller drive shaft includes threads complementary to the impeller central aperture threaded portion and wherein the relative rotation causes the threaded portion of the impeller central aperture to engage with the threaded portion of the impeller drive shaft. The relative rotation of the impeller with respect to the impeller drive shaft is continued until at least one impeller arm contacts the frusto-conical screen. Once an impeller arm contacts the frusto-conical screen, the impeller is rotated in relationship to the impeller drive shaft to cause the impeller central aperture threaded portion to disengage at least partially from the impeller drive threaded portion to form a gap between the impeller arm and the frusto-conical screen.